1. Field of the Invention
The present invention relates to a high-luminance type light-emitting diode (LED) with a heat dissipation function.
2. Related Art Statement
Light-emitting diodes (LEDs) that have been used as a backlight source for the liquid crystal panels of cell phones and other devices are characteristically small in size with a long life as a light source. In recent years, various types of LEDs have been commercialized, including one that emits colored light and one with high output that illuminates a relatively wide area at high brightness. These colored light illuminating LEDs and high output type LEDs consume a large amount of electricity and therefore heat dissipation becomes an important issue.
Generally, LEDs have an almost proportional relation between the driving current and the brightness of emitted light within a predetermined operational range. Thus, to produce a high level of brightness requires only an increase in the driving current. However, increasing the driving current results in a proportionally larger power loss in the LED element. Most of this power loss is transformed into thermal energy, which in turn increases the temperature of the LED element itself. Since the LED element has the characteristic that its light emission efficiency (current-light conversion efficiency) increases as the temperature lowers, the problem arises that the brightness of light to be produced decreases as the temperature inside the LED element increases. The service life of the LED also becomes shorter as the temperature of the LED element increases. Another problem is that heat-induced discoloration of the light-transmitting resin material sealing the LED element degrades its transparency. These problems have made it difficult to realize the commercialization of LEDs that meet both the requirements of high output and high reliability such as longevity.
To solve these problems, it is essential to provide a means of heat dissipation for LED elements. There are some proposals for such heat dissipation. One such example is the LED shown in FIG. 8 (Japanese Patent Disclosure No. 11-307820). This LED 1 comprises a pair of conductive members 2a, 2b with heat conductivity, an insulating member 3 secured to and electrically separating the conductive members 2a, 2b from each other, an LED element 4 mounted on the conductive members 2a, 2b inside the insulating member 3 at a recessed portion 3a where the conductive members 2a and 2b are exposed, and a light-transmitting seal member 5 sealing the LED element 4. The LED element 4 is mounted straddling the paired conductive members 2a, 2b exposed inside the recessed portion 3a, with the conductive members 2a, 2b soldered respectively at one end to electrode patterns 6a, 6b formed on a printed circuit board 7 such as a mother board.
Japanese Patent Disclosure No. 2002-252373 discloses an LED that employs another means of heat dissipation. In this LED, a substrate on which the LED element is mounted and a lead frame serving as terminal electrodes are formed from the same material, and the substrate situated at almost the same level as the bottom surface of the lead frame is directly mounted on a printed circuit board of an electronic device for example, when the LED is used in an electronic device.
However, in the LED 1 shown in FIG. 8, since heat generated by the LED element 4 is dissipated through the pair of conductive members 2a, 2b and the electrode patterns 6a, 6b to the printed circuit board 7, the heat dissipating performance depends on the heat conductivity of the printed circuit board 7. For example, when a metal core substrate with an excellent heat conductivity is used for the printed circuit board 7, a good heat dissipating performance can be expected, whereas the use of an ordinary printed circuit board made of a low-cost material such as glass epoxy cannot be expected to produce a significant heat dissipating effect. This is because the heat conductivity of glass epoxy materials is a few hundred times smaller than that of metal materials such as copper alloy and the resulting large thermal resistance prevents the heat from being released efficiently. Thus, to realize an efficient heat dissipation, it is essential that a metal core substrate is used for the printed circuit board 7. The metal core substrate, however, has the problems of increased cost and that the metal core substrate is difficult to wire on both sides for high density integration. Further, since the metal core substrate is a conductive material, it must be insulated by covering its surface with an insulating layer, which in turn degrades the heat conduction and therefore the heat dissipation effect.
The LED disclosed in Japanese Patent Disclosure No. 2002-252373 also has a similar problem. That is, since the substrate is mounted in intimate contact with the printed circuit board, the heat conduction from the substrate to the printed circuit board is relatively good. But when the printed circuit board is made of glass epoxy, its low heat conductivity worsens the heat dissipating performance.